An embodiment of the present invention relates to a valve timing control device or cam phaser of an internal combustion engine, wherein the cam phaser is configured to automatically locate to its mid-lock position, without having to rely on electronic control.
At least one embodiment of the present invention also relates to a cam phaser which is configured to use cam torque to recirculate oil to assist the oil pressure in locating the rotor to its mid-lock position.
A typical internal combustion engine provides that a crankshaft drives a drive wheel using a chain or drive belt. A stator is joined in a torsionally rigid manner to the drive wheel. As such, the stator is drive-connected to the crankshaft by means of this drive element and drive wheel.
A corresponding rotor is engaged with the stator, and is joined to the camshaft in a torsionally rigid manner. The camshaft has cam lobes thereon which push against gas exchange valves in order to open them. By rotating the camshaft, the opening and closing time points of the gas exchange valves are shifted so that the internal combustion engine offers its optimal performance at the speed involved.
To optimize performance during operation of the internal combustion engine, the angular position of the camshaft is continuously changed relative to the drive wheel depending on the relative position of the rotor relative to the stator. Specifically, the engine RPM and the amount of torque and horsepower the engine is required to produce are the bases for the timing adjustments. These adjustments take place while the engine is in operation. This makes variable valve timing possible because intake and exhaust valve timing is constantly adjusted throughout the RPM range. The performance benefits include the increase of engine efficiency and improvement of idle smoothness. The engine can also deliver more horsepower and torque versus a similar displacement engine with conventional valve timing. This also allows the engine to have improved fuel economy and results in the engine emitting fewer hydrocarbons.
The stator includes webs which protrude radially toward a central axis of the stator. Intermediate spaces are formed between the adjacent webs, and pressure medium is introduced to these spaces via a hydraulic valve. The rotor includes vanes which protrude radially away from the central axis of the rotor, and project between adjacent webs of the stator. These vanes of the rotor subdivide the intermediate spaces between webs of the stator into two pressure chambers (often referred to as “A” and “B”, respectively). In order to change the angular position between the camshaft and the drive wheel, the rotor is rotated relative to stator. For this purpose, depending on the desired direction of rotation each time, the pressure medium in every other pressure chamber (“A” or “B”) is pressurized, while the other pressure chamber (“B” or “A”) is relieved of pressure toward the tank.
During some operating states of the internal combustion engine, it becomes imperative to lock the position of the rotor relative to the stator. For this purpose, a valve timing control apparatus in the form of a lock pin may be utilized on the rotor for locking into a corresponding bore.
Some systems are configured such that this bore is provided at one end or the other of the rotor's range of motion relative to the stator (proximate one web or the other). Typically, it is at the fully retarded position at which the lock pin is configured to lock. Regardless, it is easier to effect locking of the rotor at one end or the other of its range of motion because then, in case of engine shut down, cam friction can be employed to move the rotor to the locked position, or a spring can even be used to overcome opposing friction to move the rotor to the locked position.
While there are advantages to providing a system which locks the rotor relative to the stator, there is difficulty in directing the rotor to a “mid-lock” position, i.e., a lock position which is not located at either one end or the other of the rotor's range of motion relative to the stator. Specifically, when a lock position is provided in between the fully retarded position and the fully advanced position (i.e., a mid-lock position), if the engine stalls or is shut down prior to controlling the rotor to the mid-lock position, conventional end-lock systems are not able to move the rotor to the correct position and lock the phaser.
U.S. Pat. No. 8,973,542 discloses a system which provides a mid-locking system which is configured to exhaust pressure medium from one side or the other of the vanes of the rotor, in order to move the rotor to its mid-lock position. The present invention is effectively an improvement over the system disclosed in the '542 patent.